1. Field of the Invention
The present invention relates to an information recording method for optical information. More specifically, the present invention relates to a high-density information recording method and a high-density information recording medium for optical information.
2. Description of the Related Arts
In recent years, not only information for computers but also information of voices, still images, moving images, and the like are digitized, and an amount of information dealt with has become extremely large. Along with this, a necessity for further increasing capacities of optical recording media for storing such information also arises. Such optical recording media include read-only information recording media, write-once information recording media in which additional recording is allowed, and rewritable information recording media in which information can be rewritten. Rewritable information recording media include magneto-optical recording media, phase change-type optical recording media, and the like. Write-once information recording media include, for example, organic dye-based optical recording media containing an organic dye in recording layers. In particular, among organic dye-based optical recording media, CD-Rs in which recording and reproduction are performed using laser light having a wavelength of approximately 780 nm are in widespread use all over the world. Further, recently, there is a sign that DVD-Rs and DVD+Rs, in which recording and reproduction are performed using laser light having a wavelength of approximately 650 nm, become widespread following CD−Rs.
A method of recording information on such a write-once optical recording medium is performed as follows: intense laser light is applied to a recording layer containing an organic dye to change optical characteristics and shapes of the organic dye, a substrate material around the organic dye, a metal reflecting film, and the like, thus generating a difference in reflectivity between an unrecorded state and a recorded state. So far, a large number of methods for optimizing strategy (waveform rule for pulse emission) have been proposed in order to obtain favorable recording qualities. As partially described in specifications of DVD-Rs and the like, a method of, in order to record information at high density, controlling timing of an edge of a recording mark by splitting incident laser light for optical recording into multiple pulses in mark length modulation recording (method in which recording is performed by modulation of mark and space signals) has been established and is in practical use for DVD-Rs and DVD+Rs (e.g. refer to Patent Document 1).
(Patent Document 1) Japanese Unexamined Patent Publication No. 2001-176072 (paragraph 0017 and the like)
Heretofore, as a method for dramatically increasing the capacity of an optical recording medium, a technology and the like for shortening a wavelength (λ) of laser light and increasing a numerical aperture (NA) of an objective lens to reduce a laser spot diameter (laser wavelength/NA) for recording and reproduction, have been performed. By such a technology, DVD-RAMs and DVD-RWs having capacities seven times those of conventional CD-Rs have been commercialized. The amount of information has a tendency to continue to increase. Accordingly, as the demand for larger capacities of optical recording media, for example, a medium having a capacity of 15 GB or more and the size of a CD, i.e. 12 cm, is earnestly being desired in order to record high-quality video information of two hours or more. In order to obtain a medium having such a large capacity, an oscillation wavelength of laser light for recording and reproduction is being further reduced. That is, instead of semiconductor laser light which has a wavelength of 640 nm to 680 nm and which is used for present DVDs, development of an optical recording medium which allows recording and reproduction using laser light having a wavelength of 405 nm is being advanced.
Furthermore, as a next-generation DVD technology, 1-7 modulation system in which data of one bit is replaced with a data sequence of seven bits to be recorded, has been proposed. That is, in the next-generation DVD technology, instead of EFM (8 to 14 modulation) used in the CD standard heretofore and 8-16 modulation system in which data of eight bits is replaced with a data sequence of 16 bits to be recorded, 1-7 modulation system is adopted in order to further improve an efficiency of recording an enormous amount of information and to respond to the demand for larger capacities of optical recording media.
Incidentally, in the case of this 1-7 modulation system, since a mark length of a 2T signal, which is a shortest mark, is approximately ⅓ of a laser spot diameter, a signal amplitude by recording and reproducing light applied to an optical recording medium is hardly ensured. Accordingly, information of the recorded 2T signal is read using a fluctuation in signal level, i.e. asymmetry.
Moreover, in the case of 1-7 modulation system, in order to perform high-density recording, for example, the mark length of a 2T signal, which is a shortest mark, is approximately ⅓ of the laser spot diameter, and the mark length of a 3T mark, which is the second smallest signal, is approximately ½ of the laser spot diameter. Accordingly, recorded information is read using the fluctuation in signal level, i.e. asymmetry, in the case of the 2T signal for which a signal amplitude cannot be ensured, whereas recorded information is read using a signal amplitude in the case of the 3T or longer signal. Thus, recording at higher density than heretofore can be performed.
In particular, in a case of a write-once optical recording medium provided with a recording layer containing organic dyes, short recording marks are generally tend to be hard to form compared to a case of a recording layer containing phase-change materials. Accordingly, in a case where an extremely small recording mark, such as the 2T signal, is formed as described above, the asymmetry does not become approximately zero, thus causing a problem that a read error of recorded information occurs.